Spatial division multiple access wireless communication systems

ABSTRACT

A method and apparatus for increasing the capacity and quality of wireless communication between a plurality of remote users and a base station is disclosed. Using measurements from an array of receiving antennas at the base station, parameters of multiple signals transmitted to the base station from a plurality of users in the same channel are calculated and used to obtain the positions and velocities of the users. The locations and other related signal parameters are used to calculate appropriate spatial demultiplexing strategies, reconstructing the individual transmitted signals from the receiver measurements and reducing interference to acceptable levels. This heretofore unavailable location information is used in solving the hand-off problem. This information is also used to calculate an appropriate spatial multiplexing strategy for simultaneous transmission of signals to users in the same channel. This can be the same as or distinct from the aforementioned receive channel. In combination, the transmit and receive systems establish multiple full-duplex links in the same channel by directively receiving and transmitting signals at the base stations only. This invention can also be implemented at the mobile sites to improve signal quality and to establish point-to-point communication links between multiple mobile users. Furthermore, implementation of both transmit and receive functions at the base station is not required. A receive-only system base station will still improve received signal quality and therefore capacity in addition to mitigating the hand-off problem.

This is a division of application Ser. No. 07/806,695, filed Dec. 12,1991, now U.S. Pat. No. 5,515,378.

FIELD OF THE INVENTION

The field of this invention relates to multiple access wirelessinformation and communication networks and, in particular, to a methodand apparatus' for exploiting information collected by arrays ofspatially distributed sensors to substantially increase the number andquality of communication channels in wireless networks by establishingspatially directive links capable of full-duplex operation innonstationary environments without increasing the amount of allocatedfrequency spectrum.

BACKGROUND

Wireless communication systems are generally composed of one or morelocal central sites, herein termed base stations, through which wirelesstransmitter/receivers gain access to a larger information network. Thebase stations service a local area wherein a number of wireless users,fixed or mobile, are located. The function of the base station is torelay messages to and from users all over the network. In cellularmobile systems, for example, this task is performed by relaying messagesto and receiving signals from a Mobile Telephone Switching Office(MTSO). A wireless user establishes a two-way (full-duplex)communication link with one or more other users also having some accessto the network by first requesting access to the network through thelocal base station. This communication is accomplished in cellularmobile communications and wireless local area computer networks (LANs),for example, by suitably modulating electromagnetic waves.

Conventional wireless communications systems requires that userstransmit signals in different frequency channels, use different codingschemes in the same frequency channels, or be transmitted innon-overlapping time intervals for the signals to be correctly received.One aspect of the present invention describes a method and apparatus forseparating multiple messages in the same frequency, code, or timechannel using the fact that they are in different spatial channels.Hereinafter, the term channel will be used to denote any of theconventional channels (frequency, time, code) or any combinationthereof. The term spatial channel refers to the new concept unique tothis the present invention.

Wireless communication is becoming an increasingly common form ofcommunication (D. Goodman, "Trends in Cellular and CordlessCommunications," IEEE Communications Magazine, June 1991), and thedemand for such service continues to grow. Examples include cellularmobile communication networks, wireless local area computer networks,wireless telephone networks, cordless telephones, satellitecommunication networks, wireless cable TV, multi-user paging systems,high-frequency (HF) modems, and more. Current implementations of thesecommunication systems are all confined to limited frequency bands ofoperation either by practical considerations or, as is more often thecase, by government regulation. As the capacity of these systems hasbeen reached, demand for more service must be met by allocating morefrequency spectrum to the particular application along with attempts toutilize the allocated spectrum more efficiently. In light of the basicphysical principle that transmission of information requires bandwidth,the fundamental limitations of a finite amount of practically usablespectrum present a substantial barrier to meeting an exponentiallyincreasing demand for wireless information transmission. Since, as hasbeen demonstrated over the last decade, the amount of practically usablefrequency spectrum can not keep pace with the demand, there is acritical need for new technology for increasing the ability of suchsystems to transfer information (D. Goodman, op. cit., G. Calhoun,Digital Cellular Radio, Artech House 1988). This invention directlyaddresses this need and is compatible with current as well as futuremodulation schemes and standards (D. Goodman, "Second GenerationWireless Information Networks," IEEE Trans. on Veh. Tech., Vol. 40, No.2, May 1991).

In conventional wireless communications systems, a base station servesmany channels by means of different multiple access schemes, the mostcommon being Frequency-Division Multiple Access (FDMA), Time-DivisionMultiple Access (TDMA), and more recently Code-Division Multiple Access(CDMA). All current systems employ FDMA wherein the available frequencybandwidth is sliced into multiple frequency channels and signals aretransmitted simultaneously, with a maximum of one per channel at anygiven time. All wireless systems also currently employ TDMA, a techniquewherein multiple users share a common frequency channel by doing so atdifferent times, in that when a user no longer requires the channelassigned to it, the channel is reassigned to another user.

In the conventional wireless communications systems, TDMA is also beingexploited on a more detailed level. Analog data such as voice isdigitized, compressed, then sent in bursts over an assigned frequencychannel in assigned time slots. By interleaving multiple users in theavailable time slots, increases in the capacity (i.e., number ofsimultaneous users) of the system can be achieved. This requiressubstantial modifications to the base station receiver hardware as wellas the mobile units themselves, however, since the current analog unitsare not capable of exploiting this technology. Consequently, a dual-modestandard, supporting both the new digital and the old analogtransmission schemes, has had to be adopted.

CDMA allows multiple users to share a common frequency channel by usingcoded modulation schemes. The technology involves preprocessing thesignal to be transmitted by digitizing it, modulating a wideband codedpulse train, and transmitting the modulated coded signal in the assignedchannel. Multiple users are given distinct codes which decoders in thereceivers are programmed to detect. If properly designed, the number ofsimultaneous users of such a system can be increased over conventionalwireless communications systems. While theoretically sound, however, thetechnology has yet to be proven. There are substantial practicalproblems with the scheme, the most important being a stringentrequirement for accurate and rapid power control of the wirelesstransmitters. Such problems may vitiate the usefulness of CDMA inwireless communication networks. Should CDMA survive, however, the SDMAconcept described herein can be applied directly to further increasecapacity and system performance.

The aforementioned techniques represent various attempts to moreefficiently pack an increasing number of signals into fixed-widthfrequency channels. These techniques do not exploit the spatialdimension when establishing channels. This invention describes how, inaddition to traditional schemes, the spatial dimension can be exploitedto significantly increase the quality of the communication links, reducethe required amount of transmitted power, and most importantly increasethe number of channels that a base station can serve without allocationof more frequency channels. This technique is hereafter referred to asSpatial-Division Multiple Access (SDMA).

In conventional wireless communications systems, exploitation of thespatial dimension is limited to what is referred to as spatial diversityand sectorization. In spatial diversity, most commonly associated withmobile systems, two antennas are employed on reception only, and the onewith the strongest signal in the bandwidth of interest is chosen forfurther processing, or some method for combining the two outputs isapplied (P. Balaban and J. Salz, "Dual Diversity Combining andEqualization in Digital Cellular Mobile Radio", IEEE Trans. on Veh.Tech., Vo. 40, No. 2, May 1991). Though this leads to a minorimprovement in the quality of the received signal, there is no increasein system capacity.

To increase the capacity of cellular systems, service providers havebeen installing more cell sites, reducing the area covered by each siteso that more users can access the system. The idea is that signals farenough away will not interfere with local sources since power dissipatesquite rapidly in space the further from the transmitter the receiver islocated. This straightforward approach to increasing capacity is,however, quite costly as the amount of cell site hardware required isproportional to the number of cell sites, which in turn is inverselyproportional to the square of the factor by which the effective radiusof each cell is decreased. In fact, the current economics of thesituation dictate that service providers bid for precious frequencyspectrum before even considering installation of new cell sites (G.Calhoun, Digital Cellular Radio, Artech House 1988). Furthermore, thisstrategy also greatly exacerbates the hand-off problem as discussedfurther on since users enter and leave cells more often when the cellsare smaller.

Sectorization is similar in spirit and is another technique forincreasing capacity by essentially making the local areas served by eachcell smaller, thus adding more cells to the network. This isaccomplished at a common location by employing directive antennas, i.e.,receiving antennas at the cell site which receive mobile transmissionsis a particular sector only. Patents related to this basic cellularconcept have been issued to Motorola in 1977 (V. Graziano, "AntennaArray for a Cellular RF Communications System," U.S. Pat. No. 4,128,740,13/1977, U.S. Cl. 179-2 EB), Harris Corporation in 1985 (M. Barnes,"Cellular Mobile Telephone System and Method," U.S. Pat. No. 4,829,554,55/1985, U.S. Cl. 379-58), NEC Corporation in 1986 (M. Makino, "MobileRadio Communications System," U.S. Pat. No. 4,575,582, C.I.P. U.S. Pat.No. 4,796,291, 3/1986, U.S. Cl. 358-58), and Sony Corporation (T.Kunihiro, "Cordless Telephone," U.S. Pat. No. 4,965,849, 9/1989, U.S.Cl. 455-34) to name just a few. With recent developments in digitaltechnology making digital transmission and reception of informationeconomically feasible, there have been a significant number of patentsin this area as well including S. Hattori, et al., "Mobile CommunicationSystem," U.S. Pat. No. 4,947,452, 10/1989, U.S. Cl. 455-33; S. Hattori,et al., "Mobile Communication System," U.S. Pat. No. 4,955,082, 1/1989,U.S. Cl. 455-33; T. Shimizu, et al., "High Throughput CommunicationMethod and System for a Digital Mobile Station When Crossing a ZoneBoundary During a Session," U.S. Pat. No. 4,989,204, 12/1989, U.S. Cl.370-94.1; T. Freeburg, et al., "Cellular Data Telephone System andCellular Data Telephone Therefor," U.S. Pat. No. 4,837,800, 13/1988,U.S. Cl. 379-59; and R. Mahany, "Mobile Radio Data Communication Systemand Method," U.S. Pat. No. 4,910,794, 6/1988, U.S. Cl. 455-67. Thoughsectorization increases capacity, it has limited potential for meetingfuture demand and is fundamentally limited by the basic physicalprinciples that do not permit the design of exceedingly small sectorswithout exceedingly large antennas. Furthermore, since sectorization issimply another method for increasing the number of cells, the hand-offproblem which is discussed in detail further on, is exacerbated.

In the aforementioned conventional systems, it is assumed that there isonly one mobile unit at a time transmitting in a given cell on a givenfrequency. Other transmitters actively transmitting in the samefrequency channel at the same time are considered to be cochannelinterference, a situation which current systems attempt to prevent sinceit leads to significant performance degradation. Cochannel interference,in fact, is a major factor in determining how often (spatially)frequency channels can be reused, i.e., assigned to different cells (W.Lee, Mobile Cellular Telecommunication Systems, McGraw-Hill, 1989). Thecochannel interference problem pervades all wireless communicationsystems, not just cellular mobile communications, and attempts to solveit in current systems have all been formulated on the premise that thecochannel signals represent disturbances to be eliminated an that onlyone antenna/receiver output is available for the task.

Conventional systems in which interference suppression is performedusing adaptive filters in the time-domain and the output of a singleantenna includes F. Gutleber, "Interference Cancelling System for aMobile Subscriber Access Communications System," U.S. Pat. No.4,434,505, 14/1982, U.S. Cl. 455-50; and Y. Shimura, "Base StationCapable of Monitoring Occurrence of Interference on Every Transmission,"U.S. Pat. No. 4,837,801, 8/1987, U.S. Cl. 379-61. These techniques arebased on an assumption of statistical stationarity, i.e., that thechannel characteristics do not change very fast. In the mobilecommunications environment where deep Rayleigh fading (40 dB) at ratesup to 200 Hz is a dominant factor, the stationarity assumption is knownto be invalid, and the performance of these conventional techniques isknown to be quite susceptible to errors in the assumptions made. Inparticular, in the presence of multiple delayed copies of the samesignal (i.e., specular multipath), these adaptive filters can null thedesired signal.

Time-domain adaptive filter techniques have also been developed toimprove channel quality for digital transmission in the presence of theaforementioned Rayleigh fading which causes intersymbol interference atthe receiver. Examples of conventional techniques for addressing thistype of interference include J. Proakis, "Adaptive Equalization for TDMADigital Mobile Radio", IEEE Trans. on Veh. Tech., Vo. 40, No. 2, May1991, and numerous other technical references in the open literature.Similar equalization techniques have been adopted in the current digitalGSM system. The foregoing systems are completely compatible with thisinvention and can be incorporated in the demodulation step as iscurrently done in practice.

More recently, investigations have been undertaken into the possibilityof combining the outputs of more than one antenna to improve signalquality by eliminating cochannel interference. In the context ofwireless LANs and PBXs, a multi-channel adaptive equalization scheme hasbeen described by J. Winter, "Wireless PBX/LAN System with OptimumCombining," U.S. Pat. No. 4,639,914, 9/1984, U.S. C. 370-110.1. Thismethod relies on code assignment (CDMA) to a known number oftransmitters and tight power control circuitry. It also requirestime-division duplexing, i.e., transmission and reception at the basestation and the wireless terminals must occur at different times on thesame frequency. This requirement results from the fact that theinformation in the spatial dimension is not being fully exploited;source locations are not calculated. The aforementioned stationarityassumption is also critical to the technique and it is therefore notapplicable to the mobile environment. Furthermore, it is modulationdependent and is designed solely for interoffice wireless LANs usingdigital transmission technology.

In the context of simply combating the cellular mobile problem ofRayleigh fading at the mobile receiver, a method of incorporating pluralantennas is also described by P. Balaban and J. Salz, op. cit.. Hereinas in similar well-known techniques, various assumptions concerning thetemporal characteristics of the signal of interest and its relationshipto the cochannel interfering signals are made and on the basis thereof,a time-varying filter is constructed with best possible reconstructionof the signal of interest as its sole purpose. The performance of thistechniques is also known to be quite susceptible to errors in theassumptions made, specifically the stationary channel assumption. Infact, mobile unit implementation of this invention mitigates theRayleigh fading problem to a large extent.

The undesirable characteristics of the aforementioned adaptivetechniques are a consequence of the fact that only assumed time-domainproperties of the received signals are being exploited, and that one ofthe signals present in the data is treated differently from theremaining signals, i.e., the cochannel interferers. It has been foundthis invention is that cochannel interferers simply represent aplurality of users attempting to access the system simultaneously on thesame channel. Accordingly, one aspect of the present invention enablesthis situation to be managed regardless of the modulation type (analogor digital) and in the presence of multiple arrivals of the same signal(i.e., specular multipath) is a significant advantage over the abovedescribed conventional techniques.

Efficient exploitation of the spatial dimension to increase capacityrequires the ability to separate a number of users simultaneouslycommunicating on the same channel at the same time in the same localarea (cell). As will be explained, one aspect of the present inventionperforms this separation by distinguishing the signals on the basis oftheir angle-of-arrival, information which is used to ascertain thelocation of the transmitters. The process of localization of thetransmitter according to this aspect of the invention providesheretofore unexpected advantages over conventional techniques.

Localization of signals in space using data collected by an array ofsensors has been accomplished in fields other than wirelesscommunications. Such is the case, for example, in tracking of aircraftand other aerospace objects using phased-array radars. Examples ofarrays with such structure include R. Roy, et at., "Methods forEstimating Signal Source Locations and Signal Parameters Using an Arrayof Signal Sensor Pairs," U.S. Pat. No. 4,750,147, 3/1985, U.S. Cl.364-800, and R. Roy, et al., "Methods and Arrangements for SignalReception and Parameter Estimation," U.S. Pat. No. 4,965,732, 7/1987,U.S. Cl. 364-460. The arrays used therein, however, are required topossess a special structure, i.e., sensors occur in pairs of identicalelements. The present invention is not limited to the use of suchspecialized array structure.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus forincreasing capacity and improving quality of wireless communicationnetworks. By appropriately collecting and subsequently processingspatially distributed sensor measurements, the spatial locations ofmultiple sources simultaneously transmitting information in a commonchannel can be estimated and the individual signal waveformsreconstructed. Using location information and an appropriately designedtransmitter array, information is simultaneously transmitted to thesources on a common channel without creating cochannel interferencewhich would otherwise compromise two-way (full-duplex) communicationlinks. Unique to this invention is the estimation of source locationsand spatial demultiplexing of multiple signals in the same channel, anda method for establishing communication between multiple transmittersand receivers on the same channel.

With regard to the cellular mobile communications marketplace inparticular, it is generally agreed that there are four areas of majorconcern that do not seem to be addressable by conventional techniques(G. Calhoun, op. cit.):

1. flaws in the coverage of the area served by the base station network,

2. flaws in the processing of the calls by the base stations and theMTSO,

3. lack of communications privacy and security, and

4. the problem of digital data transmission.

The consensus among those skilled in the art of communication systemsdesign is that a technological breakthrough is required to solve thesecritical problems. Interestingly the capacity problem is not mentionedin this particular list, though it is the major theme of the entiremonograph (G. Calhoun, op. cit.). This invention is a technologicalbreakthrough that directly addresses the first three issues, iscompatible with any of the potential solutions to the fourth, anddirectly addresses the major concern, that of increasing capacity ofcurrent wireless communication networks without increasing allocatedbandwidth.

Flaws in the area coverage are simply due to the fact that cellularsystems are currently constructed on the basis of a simple geometricmodel of coverage area, e.g., the area covered by a particular basestation is a symmetric geometrical object collections of which arecapable of tiling a large geographic area with a minimum of overlap. Thehexagon is the primary example. Unfortunately, this is a deficientdesign strategy which takes into account neither the topology of theurban area (configuration of skyscrapers, hills, areas of dense foliage,etc.) nor the non-uniform distribution of potential users of the system,e.g., freeways which become parking lots at rush hour, delaying people'sschedules and creating a demand for mobile communication services highlyconcentrated in one particular area or hot spot. These issues are nowknown to be exceedingly important in the proper design of cell sitenetworks, but conventional techniques do not indicate a good solution.Cells do not have well-defined boundaries and the real-worldirregularities can seriously reduce the efficiency of the cellularsystem. An AT&T study (J. Whitehead, "Cellular System Design: AnEmerging Engineering Discipline," IEEE Communications Magazine, Vol. 24,No. 2, February 1986, p. 10) concluded that "irregular traffic, terrain,and growth situations limit the spectrum efficiency of orthodox[cellular designs] to about half the ideal".

One aspect of this invention overcomes the problem of area coverage. Byappropriately processing the outputs of multiple antennas at the basestations, an increase in effective receiver gain is achieved and signalsof significantly lower power than those receivable in the conventionaldevices are detected and capable of being processed. This directlymitigates the problem of signal drop-out to a large extent. Furthermore,with cooperating base stations, several base stations can service thesame geographic area during periods of peak usage by dynamic allocationof channels among base stations, thereby eliminating hot spots when andwhere they occur.

Flaws in the processing of calls are a major concern to the cellularindustry. The basic problem arises due to the fact that calls must betransferred in a process termed hand-off from one base station toanother as the mobile unit moves from one cell to an adjacent cell. Theproblem with current cellular systems is that the location of the mobileunit is unknown and clearly therefore neither is its direction of motion(or for that matter even whether it is moving or stationary). Withoutthis information, the cellular system has no idea to which cell thehand-off should be performed. Current systems rely solely on thereceived power levels to ascertain whether to attempt a hand-off, and towhich cell the call or link should be assigned (G. Huensch, et al.,"High Density Cellular Mobile Radio Communications," U.S. Pat. No.4,475,010, 27/1983, U.S. Cl. 179-2 EB). Due to the aforementionedirregularities in area coverage, there is a significant probability thatthe call will be handed to an inappropriate cell, one sufficiently faraway that once the mobile unit changes location by a small amount, theassignment error becomes obvious.

According to some cellular operators, this is a substantial problem. Thecellular system in Los Angeles has experienced "considerable problemswith crosstalk. You get to a high elevation, your own signal weakens andyou start hearing another conversation. Right after that, you lose thecall." (S. Titch, "For PacTel Mobile, Bigger is not Better",Communications Week, Jan. 27, 1986, p. 54). The former director ofcellular development for Motorola has stated that "anywhere from 10% to25% of the time the system made decision errors when a subscriber movedfrom one cell to another. When the switch made an error it assigned thesubscriber's call to a distant cell site rather than the closest one.The system would then start serving a different subscriber on the samefrequency in the same cell as the first subscriber. The result wascellular disaster. When this situation occurs, subscribers hear othersubscribers conversations on their channels. They get cutoff or, if theyare lucky [?], the channel gets noisy. This problem happens often enoughto be infuriating." (M. Cooper, "Cellular Does Work--If the System isDesigned Correctly," Personal Communications, June 1985, p. 41) Inaddition to the crosstalk problem, mounting evidence indicates localvariations in signal strength cause significantly higher hand-off ratesthan predicted based on hard-cell boundaries, and this increased load onthe central switch is cause for concern that the cellular systemcapacity may be reduced.

One aspect of this invention overcomes the problem of flaws in callprocessing. By providing estimates of the locations and velocities ofthe mobile units heretofore not available, intelligent hand-offstrategies can be instituted. This directly meets the need of thecellular operators to know the location and velocity of the mobile unit.Coupled with the increased capability to process lower power signals,the foregoing aspects of the present invention achieve a significantimprovement in the performance of cellular communication systems.

Privacy is another issue of great concern to the cellular industry. Theinsecure nature of the current systems (anyone can legally listen to allmobile channels) is a critical factor impacting sales of the system tocustomers involved in sensitive business transactions as well as manygovernment users. Cellular operators are realizing that some positivesolution must be found (G. Calhoun, op. cit.). The root of the problemis that in current systems, the signals are transmitted bynon-directional antennas as uniformly as is possible over the region orsector they serve, and can be intercepted by receivers locatedpractically anywhere nearby. One solution to this problem is encryption.However, this requires eitheranalog-to-digital-(de-)encryption-to-analog hardware at both the cellsite and the mobile unit, or conversion to a new digital standard, bothof which are rather costly solutions.

Another aspect of this invention provides a significant improvement inmaintaining privacy. Utilizing knowledge of the location of the intendedreceiver, the base station transmitter is designed so as to transmit theintended signal essentially in that direction only. By transmitting thesignal in only one direction, eavesdropping is mitigated to a largeextent. To intercept the signal, the eavesdropper must be in the samegeographical region, a condition which will occur rarely in practice.The extent to which the transmitted signal from the base station to themobile unit can be localized in space is a direct function of the numberof transmitting antennas available. As is well-known in conventionaldevices such as phased-array antennas, so-called beamwidths oftransmitted energy depend directly on the antenna aperture and thenumber and location of the transmitting antennas. Thus, system securityfrom the base station to the mobile unit can be easily increased at thecost of increasing the transmitter hardware at the base stations.Although not required in this invention, the same system can be employedat the mobile unit: employing multiple receive and transmit antennas tolocate the (electrically) closest base station and transmit selectivelyin that direction. The drawback is the significant increase in mobileunit complexity.

The lack of compatibility of current analog cellular systems withdigital data transmission is not an issue of major concern presently.Most users are concerned with voice transmission and the currentlyallocated bandwidths (30 KHz per channel) are sufficient for thatpurpose. They do, however, fundamentally limit the amount of data whichcan be successfully transmitted to about 10 kb/s theoretically, and toabout 1200 b/s practically due to the poor quality of the channel. Asusers' demands increase, so will the need for high-speed datatransmission over the cellular network and the current system will haveto be revised.

In this regard, another aspect of this invention is independence of thissignal modulation scheme. It works equally well with digital or analogmodulation of the source signals. In fact, one of the major concerns inapplying digital spread-spectrum techniques in the cellular mobileindustry is that of intersymbol interference from multipath reflections.This problem is mitigated by the present invention since by employingmultiple antennas, the different directions-of-arrival associated withdifferent paths from the same source can be detected, and spatialisolation of the multiple arrivals therefrom performed. A second concernwith digital modulation is the need to maintain as large asignal-to-noise ratio as is possible to keep bit error rates as low aspossible. By employing this invention, the strength of received signalsat the base station can be significantly improved over existing systemsfor the same amount of transmitted power thus providing a significantimprovement in system performance.

Since in conventional systems the locations of the wireless units arenot known, uniform, non-directional (in azimuth) transmission from thecell sites to the wireless units is the only reasonable strategy. Uniqueto this invention, however, is the ability to estimate the location ofmultiple transmitters in the same channel. This information, heretoforeunexploitable, is used in the present invention for designing efficientstrategies for multiple signal cochannel transmission from the cell siteto the wireless unit. Well-known in the conventional systems is thedesign of antenna arrays to selectively transmit energy in prescribeddirections as a function of frequency (H. Rosen, "Steered-Beam SatelliteCommunication System," U.S. Pat. No. 4,972,151, 9/1985, U.S. Cl.342-354). As an unexpected result of this invention, spatially selectivetransmission of multiple cochannel signals from cell site to wirelessunits is made possible. In addition, the amount of power transmitted indirections other than that of the intended receiver is minimized,further mitigating the problem of cochannel interference. Another uniqueaspect of the present invention is the design of robust techniques forperforming this task.

In summary, this invention addresses key issues and problems facing thecellular mobile communications industry as well as other wirelesscommunication networks by essentially restoring the property of wirelineservice, that of point-to-point communication, lost when wires areeliminated in favor of wide-area (omni-directional) transmission andreception of (electromagnetic) radiation. No attempt is made inconventional techniques to:

1. exploit information collected by an array of sensors for the purposeof detecting and estimating the location of multiple signals on the same(frequency) channel at the same time,

2. simultaneously estimate all transmitted signals, or

3. use spatial information to simultaneously selectively transmitdifferent signals to one or more users on the same (frequency) channel.

The foregoing processes are unique to this invention and yield usefulnew and unexpected results in wireless communication networks. Althoughthe improvements achieved by the invention may increase hardwarecomplexity, such costs are easily offset by the attendant improvement inperformance and capacity. Furthermore, since the invention need not beemployed at the mobile units, the cost of increasing the quality andcapacity of current wireless networks can be kept to a minimum, thoughfurther improvement in system performance can also be realized by mobileunit implementation of the invention.

Accordingly, this method and apparatus has the following advantages overcurrent technology:

1. the invention will allow simultaneous use of any conventional(frequency, time or code) channel by multiple users, none of whichoccupy the same location in space, thereby increasing the capacity ofcurrent wireless information networks,

2. the invention provides for tracking of mobile sources, mitigating thehand-off and signal mangement problems present in the conventionalmobile cellular communication systems,

3. the invention is independent of the particular signal modulation typeand therefore compatible with current and future modulation schemes inwireless communication systems,

4. the invention provides improved signal quality at both transmittersand receivers,

5. the invention provides for improved communication security bytransmitting signals only in preferred directions thereby limiting theamount of unintentional radiation,

6. the invention allows a decrease in transmitter power to be effectedat the cell site by directive transmission while still improving signalquality,

7. the invention significantly decreases signal degradation due tocochannel interference thereby allowing frequencies in adjacent cells tobe re-used more frequently, further increasing system capacity,

8. mobile unit implementation of the invention can be effected with manyof the aforementioned advantages applying to the mobile unit as well.

Though the advantages above have been described in the context ofwireless communications, there are a number of applications in otherareas. This invention, for example, can be used as a diagnosticmeasurement device for ascertaining the quality of cell site coverage.Herein, selective transmission is not required; the receive system istransported through the coverage area and the strengths anddirections-of-arrival of signals transmitted from the base station aremonitored. There are no known systems currently available for performingthis function. The information thus obtained is important in assessingthe quality of service from proposed cell site locations as well.Further objects and advantages will become apparent from a considerationof the drawings and ensuing detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of conventional multiple wireless units successfullytransmitting and receiving on different channels.

FIG. 2 is a graphic illustration of cochannel interference resultingfrom multiple wireless units transmitting on the same channel, acritical factor in limiting the capacity of current wirelesscommunication systems.

FIG. 3 is a graphic illustration of cochannel interference resultingfrom broadcast transmission of multiple signals on the same channel tomultiple wireless units, a critical factor in limiting the capacity ofcurrent wireless communication systems.

FIG. 4 is a block diagram of the SDMA system successfully receiving andtransmitting multiple signals in one channel in accordance with theinvention, thus achieving an increase in capacity by allowing multipleusers to access one channel simultaneously.

FIG. 5 is a breakdown of the SDMA signal processor (SDMAP) in accordancewith the invention.

FIG. 6 is a graphic illustration of multiple cochannel signal receptionat the central site in accordance with the invention.

FIG. 7 is a breakdown of the SDMA multi-channel central site receiver inaccordance with the invention.

FIG. 8 is a graphic illustration of multiple cochannel signaltransmission from the central site in accordance with the invention.

FIG. 9 is a breakdown of the SDMA multi-channel central site transmitterin accordance with the invention.

FIG. 10 is a graphic illustration of multiple SDMA processors employedto increase base station capacity in accordance with the invention.

FIG. 11 contains DOA tracking and signal copy results for two nearlycoincident moving FM transmitters in a severe Rayleigh fadingenvironment.

FIG. 12 illustrates DOA tracking of FM transmitters crossing tracks in asevere Rayleigh fading environment.

FIG. 13 illustrates the compatibility of the SDMA concept with proposedCDMA technology, successfully locating and spatially demultiplexingthree spread-spectrum digital transmitters.

FIG. 14 illustrates the effectiveness of the robust SDMA spatialmultiplexing scheme, unique to this invention, for directivetransmission of signals to wireless receivers on the same frequency.

REFERENCE NUMERALS IN DRAWINGS

20. mobile unit 1 transmitter and receiver

22. mobile unit 2 transmitter and receiver

24. mobile unit d transmitter and receiver

26. conventional multi-channel receiver

28. channel 1 output of the conventional multi-channel receiver

30. channel 2 output of the conventional multi-channel receiver

32. channel d output of the conventional multi-channel receiver

34. channel 1 input to the conventional multi-channel transmitter

36. channel 2 input to the conventional multi-channel transmitter

38. channel d input to the conventional multi-channel transmitter

40. conventional multi-channel transmitter

42. conventional multi-channel receivers

44. conventional multi-channel receiver outputs

46. spatial demultiplexers

48. Spatial Division Multiple Access signal Processor (SDMAP)

50. spatially demultiplexed signals

52. signal demodulators

54. demodulated signals to switching network

56. wide area network

58. switching network

60. signals from switching network

62. signal modulators

64. modulated signals to be transmitted

66. spatial multiplexers

68. spatially multiplexed signals to transmitters

70. conventional multichannel transmitters

72. central controller

74. spatial multiplexer control signals

76. spatial demultiplexer control signals

78. central controller/SDMAP communication link

100. spatially combined signals received at base station

102. multi-channel receiver for antenna 1

104. multi-channel receiver for antenna 2

106. multi-channel receiver for antenna m_(r)

112. channel 1 output from receiver 1

114. channel 1 output from receiver 2

116. channel 1 output from receiver m_(r)

120. SDMAP and spatial demultiplexer

122. spatial demultiplexer output 1

124. spatial demultiplexer output 2

126. spatial demultiplexer output d

132. signal 1 input to channel 1 spatial multiplexer

134. signal 2 input to channel 1 spatial multiplexer

136. signal d input to channel 1 spatial multiplexer

138. SDMAP and spatial multiplexer

142. signal input to transmitter 1 channel 1

144. signal input to transmitter 2 channel 1

146. signal input to transmitter m_(t).sbsb.χ channel 1

152. multi-channel transmitter for antenna 1

154. multi-channel transmitter for antenna 2

156. multi-channel transmitter for antenna m_(t).sbsb.χ

160. data compressor

162. signals to signal detector from data compressor

164. signal detector

166. signals to source tracker from data compressor

168. signals to parameter estimator from signal detector

170. parameter estimator

172. signals to source tracker from parameter estimator

174. source tracker

176. source tracker outputs

178. spatial demultiplexer controller

180. spatial multiplexer controller

182. tracker outputs to central controller

184. central controller signals to SDMAP

190. base station 1

192. base station 1 link to base station supervisor

194. base station 2

196. base station 2 link to base station supervisor

198. base station n_(b) link to base station supervisor

200. base station n_(b)

202. SDMA processor for channel 1

204. SDMA processor for channel 2

206. SDMA processor for channel n

220. base station supervisor

DETAILED DESCRIPTION

FIG. 1 shows an example of current wireless communication networks.Wireless transmitter/receiver units (20,22,24), for the purposes ofillustration shown as vehicular mobile units, are assigned to distinct(frequency) channels and thereby allowed to communicate simultaneously.A multi-channel receiver (26) exploits the fact that they are ondifferent frequency channels to correctly separate the signals(28,30,32) which are then subsequently demodulated and passed along tothe rest of the network. A multi-channel transmitter (40) transmitssignals (34,36,38) to the wireless units (20,22,24) in another set ofdistinct frequencies. For example, in current cellular mobilecommunication systems, mobile units receive transmissions from basestations in channels 45 MHz above those frequency channels they transmitinformation to the base stations. This allows for simultaneoustransmission and reception of information at both the base station andmobile units.

FIG. 2 shows a disadvantage of current wireless communication systems.Wireless units (20,22,24) transmitting on the same conventional channel(the same carrier frequency ƒ_(c1) in this diagram) can not be resolvedat the receiver (26) due to the fact that there is no way in currentsystems to distinguish one signal from the other when they share thesame channel. The receiver output (28) is a combination of all signalspresent in the channel even after down-conversion to baseband frequencyƒ_(b).

FIG. 3 shows a similar disadvantage of current wireless communicationsystems with respect to communication from the base station transmitter(40) to the remote receivers. The function of the multi-channeltransmitter is to up-convert signals from baseband frequency ƒ_(b) toone of the multi-channel carrier frequencies for transmission to themobile unit. Wireless units (20,22,24) on a particular channel (the samecarrier frequency ƒ_(c1) in this diagram) receive a combination ofmultiple signals transmitted from the base station transmitter (40) inthat frequency channel (34). This is due to the fact that there is nomethod in current systems for preventing all signals transmitted in thesame frequency channel from reaching all receivers in a given cell orsector thereof set to receive signals in that particular channel.Signals received at the wireless units are combinations of all signalstransmitted in that channel.

FIG. 6 is an illustration of the method used by this invention toovercome the aforementioned multiple signal reception problem at one ormore base stations. Multiple signals from wireless units (20,22,24)transmitting in the same channel are received by an array of sensors andreceivers (42). These cochannel signals are spatially demultiplexed by aspatial demultiplexer (46) which is controlled by a Spatial DivisionMultiple Access signal Processor (SDMAP) (48). The demultiplexed signals(50) are then sent to signal demodulators (52) as is well-known in priorart.

FIG. 8 is an illustration of the method used by this invention toovercome the aforementioned multiple signal reception problem at themobile wireless unit. Multiple signals (64) from signal modulators,assumed therein as all being in the same frequency channel forillustrative purposes, are appropriately combined by a spatialmultiplexer (66) under control of the SDMAP (48) so as to eliminate allcochannel interference at the wireless units (20,22,24). These signals(68) are sent to multichannel transmitters (70) and subsequentlytransmitted by an array of antennas to wireless units (20,22,24). Asindicated in the illustration, by appropriate design of the spatialmultiplexer, wireless unit (20) receives none of the signal beingtransmitted to units (22) or (24), and similarly for the other twounits. In conjunction with FIG. 6, a plurality of full-duplex links arehereby established. The capability to establish more than onefull-duplex link simultaneously in the same (frequency) channel isunique to the SDMA invention.

FIG. 4 shows a block diagram of one embodiment of an SDMA systemsuccessfully receiving multiple signals in one channel and transmittingmultiple signals in another channel by using different spatial channels.The intent of the figure is to indicate that these messages arebroadcast on the same (frequency) channels, from the wireless units tothe base-station at ƒ_(c1) and from the base station to the wirelessunits at ƒ_(c2), at the same time. This is a situation heretofore notallowed since the messages interfere with each other in current systemsas indicated in FIG. 2 and FIG. 3. Signals transmitted in the samechannel by wireless units (20,22,24) are received at the base station bymultiple antennas. The output of each of the antennas is sent to amultichannel receiver as is the practice in current systems for a singleantenna. The m₂ antennas can be individual antennas or a multiple-feedsingle-dish antenna as is well-known. Herein each feed of amultiple-feed single-dish antenna is referred to as an antenna.

The multichannel receiver takes an antenna input and has one output foreach frequency channel which it is capable of processing. For example,in current analog cellular systems, the receiver consists of a bank ofbandpass filters, one such filter tuned to each of the frequencychannels assigned to that base station. In one embodiment of thisinvention, one such receiver is assigned to each antenna as shown inFIG. 7 (102,104,106). In another embodiment, several antennas areswitched via a high-speed switching circuit to a single receiver. Theoutput of the multichannel receivers for a particular (frequency)channel is a plurality of signals (112,114,116), one signal from thatchannel for each antenna/receiver pair. These signals are processed as agroup by the SDMAP/Spatial Demultiplexer (120) so as to recover theoriginal transmitted signals (122,124,126). Though the diagram impliesthat a single SDMAP and spatial demultiplexer is dedicated to eachchannel, in another embodiment several channels are multiplexed onto asingle SDMAP and spatial demultiplexer.

Referring back to FIG. 4, in one embodiment, receiver outputs (44) aredigitized after down-conversion to baseband in the multichannelreceivers (42) and transmitted in digital form to SDMAPs (48) andspatial demultiplexers (46). The outputs of the spatial demultiplexers(50) are, in one embodiment, demodulated digitally and converted toanalog for transmission through the switching network (58), and inanother embodiment, converted to analog prior to demodulation. In yetanother embodiment of this invention, A/D conversion of the analogreceiver outputs (44) is performed in the SDMAPs, and analog receiveroutputs (44) are sent to spatial demultiplexers (46) where digitallycontrolled analog weight-and-sum circuits spatially demultiplex theanalog receiver outputs (44) and analog outputs (50) are sent to analogdemodulators (52).

Generally, a function of the SDMAP (48) is to calculate appropriatecontrol signals for the spatial demultiplexer (46) and spatialmultiplexer (66) by processing the information received from multichanelreceivers (42) and information provided by the SDMA controller (78). TheSDMAP also sends tracking and other signal parameter information to theSDMA controller (72) for use in channel assignment and intelligenthand-off. A detailed description of the SDMAP is given below.

Spatial demultiplexers (46) demultiplex the outputs (44) of themultichannel receivers (42). This function is performed for each(frequency) receive channel assigned to the cell site. In oneembodiment, in each channel, the signals (44) are appropriately combinedby the spatial demultiplexer to provide one output for each signalpresent in that channel (C1 in FIG. 7). Herein, appropriately combinedis defined to be combined so that the signal from each wireless unit ina channel appears at the appropriate output of the spatialdemultiplexer. This is a unique aspect of this invention.

The outputs (50) of the spatial demultiplexer (46) for a particularchannel are the separated signals transmitted from the wireless units tothe base station in that channel, and are sent to demodulators as isdone in current system. The demodulated signals are then routed througha switching network (58) to their appropriate destination as iscurrently done.

Signals destined for the wireless units are obtained from the sameswitching network (58) and directed to signal modulators (62) as in donein current systems. Modulated baseband signals (64) are sent to spatialmultiplexers (66) where they are appropriately processed as directed bythe SDMAP (48) for transmission to the wireless units. In thisillustration, these wireless units are assumed to be the same as thosewhose signals were received in the receivers (42). This need not be thecase and is not a restriction of the current invention; they are hereinthe same for illustrative purposes only.

Multichannel transmitters (70) similar in structure to the receivers(42) are employed, there being one transmitter for each of them_(t).sbsb.χ transmitting antennas as shown in FIG. 9 (152,154,156).Each transmitter appropriately combines the outputs of each channelassigned to the base station for the purpose of transmission of thesignals through the associated antenna to the wireless units as incurrent systems.

The function of the SDMAP/spatial multiplexer (138) shown in FIG. 9 isto multiplex one or more signals (132,134,136) into a particular channel(C1 in FIG. 9), but different spatial channels. The SDMAP/spatialmultiplexer (138) appropriately combines the signals (132,134,136) andprovides one output for the particular channel (C1 in FIG. 9) in eachtransmitter (152,154,156). Herein, appropriately combined is defined tobe combined so that each wireless unit receives only the signal intendedfor it. No other signals arrive at that particular wireless unitreceiving in that (frequency) channel. This is a unique aspect of theinvention.

Spatial multiplexing is performed for each channel (C1, C2, . . . , Cnin FIG. 9). In one embodiment, a separate spatial multiplexer isprovided for each channel. In another embodiment, the multiplexing taskfor several channels is performed by the same multiplexer hardware. Ifthe signals (64) from the signal modulators (62) are analog, in oneembodiment the spatial multiplexers (66) are composed of digitallycontrolled analog components. In another embodiment, the signals (62)are digitized if necessary, appropriately combined in the spatialmultiplexers (66), then sent to the multichannel transmitters (70) forD/A conversion and transmission to the wireless units.

The Spatial Division Multiple Access Signal Processor (SDMAP)

FIG. 5 shows a breakdown of a Spatial Division Multiple Access signalProcessor (SDMAP) (48). The function of the SDMAP includes determininghow many signals are present in a particular channel, estimating signalparameters such as the spatial location of the transmitters (i.e.,directions-of-arrival DOAs and distance from the base station), anddetermining the appropriate spatial demultiplexing and multiplexingschemes. Inputs (44) to the SDMAP include outputs of base stationreceivers, one for each receiving antenna. In one embodiment, thereceivers perform quadrature detection of the signals as in currentsystems, in which case there are in-phase (I) and quadrature (Q)components (signals) output from each channel behind each antenna. Inanother embodiment, a single down-converted component, I or Q or anycombination thereof, is used. In one embodiment, the receivers digitizethe data before passing it to the SDMAP. In another embodiment,digitization is performed in the data compressor (160) asaforementioned.

In one embodiment of the invention, the SDMAP accomplishes its task byfirst obtaining estimates of important signal related parameters such astheir directions-of-arrival (DOAs) without exploiting temporalproperties of the signal. This is appropriate, for example, insituations where analog modulation schemes are employed and there islittle is known about the signal waveform. In a second embodiment, knowntraining sequences placed in digital data streams for the purpose ofchannel equalization can be used in conjunction with sensor arrayinformation to calculate signal parameter estimates such as DOAs andsignal power levels. This information is then used to calculateappropriate weights (76) for a spatial demultiplexer implemented in thisembodiment as a linear combiner, i.e., a weight-and-sum operation. In athird embodiment, TOA-related parameters from the parameter estimatorare used in conjunction with signal correlation parameters to ascertainwhich signals are multipath versions of a common signal. Relative delaysare then calculated such that the signals can be coherently combined,thus further increasing the quality of the estimated signals. Theability to exploit sensor array information in this manner is unique tothis invention.

However, in another embodiment of this invention, the function of thespatial demultiplexer is performed in conjunction with the estimation ofother source parameters such as the DOAs. As an example of one suchembodiment of this type, the constant modulus property (i.e., constantamplitude) of various communication signals such as digitalphase-shift-keyed (PSK) and analog FM waveforms can be exploited alongwith properties of the array of receiving antennas to simultaneouslyestimate the source waveforms as well as their DOAs using multichannelconstant-modulus algorithms (CMA) which are well-known in the prior art.

In another embodiment, extended Kalman filters, also well-known in theprior art (C. Chui and C. Chen, Kalman Filtering with Real-TimeApplications, Springer-Verlag, 1991), can be used to exploit these andsimilar properties. In these and similar embodiments, the function ofthe spatial demultiplexer (46) is assumed in the SDMAP (48), and theoutputs of the SDMAP (76) are the spatially demultiplexed signals to besent to the demodulators.

Referring again to FIG. 5, data compression (160) is performed to reducethe amount of data, and in one embodiment consists of accumulation of asample covariance matrix involving sums of outer products of the sampledreceiver outputs in a particular channel. Hereafter, these sampledoutputs are referred to as data vectors, and there is one such datavector at each sample time for each of the channels assigned to aparticular base station. In another embodiment, the compressed data aresimply the unprocessed data vectors. If I and Q signals (44) are outputfrom the receivers, each data vector is a collection of m_(r) complexnumbers, one for each of the m_(r) receiver/antenna pairs.

In a third embodiment, data compression also includes using known signalinformation such as training sequences present in wireless digitalsystems (D. Goodman, "Second Generation Wireless Information Networks,"IEEE Trans. of Veh. Tech., Vol. 40, No. 2, May 1991) and mobile unittransponder responses in current analog systems to calculatetime-of-arrival (TOA) of a distinct periodic signal feature, a parametercontaining valuable information related to the distance between cellsites and the wireless transmitter which is exploited in thisembodiment.

Compressed data (162) are passed to a signal detector (164) fordetection of the number of signals present in the channel. In oneembodiment, statistical detection schemes are employed in conjunctionwith information from a SDMA controller (72) to estimate the number ofsources present in the channel. This information and the (compressed)data (168) are sent to a parameter estimator (170) where estimates ofsignal parameters including those related to the source locations (e.g.,DOAs and range) are obtained.

Location-related parameter estimates (172) are passed to a sourcetracker (174). In one embodiment, the function of the source tracker isto keep track of the positions of each of the transmitters as a functionof time. This is implemented by known nonlinear filtering techniquessuch as the aforementioned extended Kalman filter (EKF). In anotherembodiment, velocities and accelerations of each of the wireless unitsin a particular channel are tracked as well. Inputs to the EKF in oneembodiment include the DOAs and TOAs from the local base station. Inanother embodiment, DOA and TOA measurements from other nearby cellsites also receiving transmissions from the mobile units areincorporated along with known locations of the cell sites to furtherimprove the estimation accuracy of the EKF as is well-known in priorart. The tracker (174) outputs are sent along with the (compressed) data(176) to a spatial demultiplexer controller (178), to control thefunction of the spatial demultiplexer, and to a spatial multiplexercontroller (180) to control the function of the spatial multiplexer.

SDMA Controller

FIG. 10 displays a SDMA controller (72) which supervises channelallocation, and a plurality of SDMA systems (202,204,206). Asaforementioned, each SDMA system receives signals (44a,44b,44c) from themultichannel receivers (42) and sends signals (68a,68b,68c) to themultichannel transmitters (70) for transmission to the wireless units.The SDMA systems also communicate (tracking) information(182a,182b,182c) as aforementioned to the SDMA controller and receiveinformation (182a,182b,182c) from the SDMA controller. Not shown in thisillustration is a link between the base stations and their access to awide area network through a switching network. Though such links arepresent in current cellular mobile networks and wireless LANs, they arecertainly not required in this invention. Point-to-point communicationbetween wireless units through the base station is possible withoutentering a wide area network.

The function of the SDMA system is performed for each channel(202,204,206), denoted CH 1, CH 2, . . . , CH n in FIG. 10, allocated toa base station for reception. In one embodiment, there is a separateSDMA system for each channel. In another embodiment, several channelsare processed in the same SDMA system.

An objective of the SDMA controller (72) is to prevent wireless unitsfrom becoming coincident in (frequency or code) channel, time, andspatial (location) space. As required, the controller instructs thewireless units to change to different (frequency or code) channels viastandard messaging schemes such as one present in current wirelesssystems.

In one embodiment, SDMA controllers at various cell sites (190,194,200)send tracking and frequency allocation information, in addition to otherrelevant source parameters such as signal power, concerning all thewireless units in their cell (192,196,198) to a base station supervisor(220). For example, in cellular mobile communications, the supervisor isthe MTSO. This information is used to mitigate the aforementionedhand-off problems present in current wireless systems. With knowledge ofthe locations and velocities of all the transmitters and knowledge ofthe areas covered by each of the cell sites, efficient and reliablehand-off strategies can be implemented.

In another embodiment, the function of the SDMA controller includesrelaying to each base station the locations and channel assignments ofcars in neighboring cells. This information is used in the spatialmultiplexer and demultiplexer controllers in the SDMAP to improve theperformance of the spatial multiplexers and demultiplexers. Furtherimprovements in capacity are also realized herein by allowing dynamicallocation of receive and transmit channels among the various cell sitesand mobile units. The ability to track multiple transmitters in wirelesscommunication networks and the significant improvements made with regardto system capacity and quality are unique to this invention.

Simulation Results

FIG. 11 illustrates the capability of the invention to simultaneouslytrack multiple transmitters in the same channel, and to spatiallydemultiplex the received signals to estimate the transmitted waveformsindividually. The receiving array is composed of a 10-element uniformlinear array of elements spaced one half-wavelength apart, i.e., 17 cmat 850 MHz. The two FM transmitters are moving toward each other andactually cross paths, i.e., the DOAs are at one point, during theinterval the same. A severe Rayleigh fading environment is simulatedwith a fade rate in excess of 100 Hz. The receiver outputs are processedin blocks of 400 data vectors (0.05 sec of data sampled at 8 KHz). Inspite of the fact that the transmitters are less than 2° apart at 1.7sec, approximately 30 m separation 1 km from the base station, theindividual signal waveforms are accurately reconstructed as shown in thelower illustration. This figure clearly manifests the efficacy of theinvention as such performance has not been achieved in current systems.The ability to separate cochannel sources in close proximity to oneanother and to successfully spatially demultiplex the received signalsis unique to this invention.

FIG. 12 is a continuation of FIG. 11 illustrating the capability of theinvention to simultaneously track multiple transmitters in the samechannel where the trajectories cross. At the midpoint of the estimationinterval, the transmitters are at the same DOA. As is easily seen, theSDMA system tracks the DOAs of the transmitters successfully. Theability to track intersecting trajectories of cochannel transmittersfrom DOA measurements made by an array of sensors is unique to thisinvention and has not been accomplished in current wireless systems.

FIG. 13 illustrates the compatibility of the SDMA concept with proposedCDMA technology. Three sources at 20°,40°, and 60° with respect to theline axis of a 10-element uniform linear array of omni-directionalantenna elements are simulated. The baud-rate rates are 1 MHz, 1 MHz,and 500 KHz respectively, and the effective signal-to-noise ratio (SNR)is approximately 0 dB. The upper plot shows the output of the firstantenna element, and the SNR is clearly seen to be nearly 0 dB, i.e.,the signal and noise amplitudes are nearly equal. The lower four smallerplots show the three spatially demultiplexed signals and the angle ofthe output of the first antenna for comparison. They clearly indicatethe ability of the SDMA system to not only spatially demultiplex theCDMA digital transmissions, but also indicate the performanceimprovement achievable. There is roughly a factor of 10 improvement inspatial demultiplexer output SNR as is quite evident. The DOA estimateswere based on only 200 snapshots, and not only was the number of signals(three) correctly detected by the SDMA detector, the estimated DOAs wereall within 0.5° of the true values. The capability to obtain estimatesof such quality, and to spatially demultiplex spread-spectrum digitalsignals in these cochannel interfering environments is unique to thisinvention.

FIG. 14 illustrates the improvement of the SDMA robust spatialmultiplexing scheme over conventional techniques. In the simulation,three transmitters were located at 40°, 50° and 90° respectively withrespect to the line axis of a 10-element λ/2-spaced uniform lineararray. The estimated directions of arrival based on 1000 data vectorswere within 0.05° of the true values, and spatial multiplexing weightvectors were computed on the basis thereof. The illustration shows theresults of spatial multiplexer design for transmission to the receiverat 90°; a design objective being the minimization of power in thedirection of the receivers at 40° and 50°. The superiority of the SDMArobust spatial multiplexer over the conventional deterministicmultiplexer which is prior art, is clearly manifest.

OPERATION OF THE INVENTION

Definitions

In the following discussion, the term base station is used to refer to asite with which (mobile) wireless units (herein termed users)communicate. Base stations are distinguished from wireless units only inthat they often have dedicated and continuous access to a broadband widearea distribution network through which many signals can be transmittedsimultaneously. This is not a restriction of the current invention asaforementioned. The term channel is used to denote any of theconventional channels (frequency, time, code) or any combinationthereof. The term spatial channel refers to the new concept unique tothe present invention.

Notation

Within the region served by a base station, users of the wireless systemsend signals to and receive signals from that base station. Denote theindividual baseband user signals by s_(r) (ω_(i), θ_(j) ^(i), t) where

ω_(i), i=1, . . . , n, denotes the i^(th) of n channels, and may denote,for example, a frequency channel in a FDMA system, a frequency-time slotin a FDMA/TDMA system, or a frequency channel and a code in a FDMA/CDMAsystem,

θ_(j) ^(i), j=1, . . . , d_(i), denotes the direction to the j^(th) ofd_(i) users, using channel ω_(i), and

t is a time index.

These baseband signals are the outputs of signal modulators which areappropriate for the modulation convention of the system as is done incurrent state-of-the-art. The inputs to these modulators are themessages users wish to send over the network. There is no restriction onthe individual messages; they can be digital or analog, data or voice.These baseband signals are up-converted, e.g., used to modulate an RFcarrier, and the resulting signals are broadcast by the usersomni-directionally in the current state-of-the-art.

In the current state-of-the-art, a different channel, ω_(i), isallocated to each concurrent user on which they transmit signals to thebase station. In current practice, a second channel is assigned forreceiving information from the base station. As is current practice,when a user's requirement for the channels ceases, they are reassigned.

The signals in the various channels are simultaneously received by thebase station and the function of the receiver is to demultiplex anddown-convert the inputs to baseband signals, s_(r) (ω₁, t), . . . ,s_(r) (ω_(n), t). Such a receiver can be viewed as having one input andn outputs. A receiver that performs this function is herein referred toas a multichannel receiver. This is graphically illustrated in FIG. 1.

Analogously, the baseband signals which are transmitted from the basestation to the users are denoted s_(t).sbsb.χ (ω_(i), θ_(j) ^(i), t)where

ω_(i), i=1, . . . , n, denotes the i^(th) of n channels, and may denote,for example, a frequency channel in a FDMA system, a frequency-time slotin a FDMA/TDMA system, or a frequency channel and a code in a FDMA/CDMAsystem, and

θ_(j) ^(i), j=1, . . . , d_(i), denotes the direction to the j^(th) ofd_(i) users using channel ω_(i).

The baseband signals in the various channels are inputs to thetransmitter which up-converts and multiplexes the signals fortransmission. Such a transmitter can be viewed as having n inputs andone output. A transmitter that performs this function is herein referredto as a multichannel transmitter. This is graphically illustrated inFIG. 1.

For the purposes of the ensuing discussion, the number of users sendinginformation to the base station is assumed to be the number of usersreceiving information from the base station. This is not a requirementof the invention. Furthermore, the n channels allocated to a basestation are, for illustrative purposes, assumed to be assigned in pairs,one for transmit and the other for receive. This is also not arequirement of the invention. In fact, in one embodiment of theinvention, fewer base station receive (user transmit) channels areallocated in favor of more base station transmit (user receive) channelsleading to a possible increase in system capacity for a fixed number ofchannels. The capability to achieve this increase in capacity is anotherunique aspect of the invention.

SDMA--The Invention

Consider a base station comprised of a collection of m_(r) antennas forthe reception of incoming signals. Such a collection is herein referredto as a receiving antenna array. Also available is a collection ofm_(t).sbsb.χ antennas for signal transmission, herein referred to as atransmitting antenna array. In general, these are two physicallydifferent arrays with different configurations and different operatingfrequencies. However, in systems where reception and transmission neednot occur simultaneously, the same array could serve as both thereceiving and transmitting antenna array. Furthermore, the inventiondoes not require transmission and reception to be on separatefrequencies.

In this invention, multiple users can be allocated to each channelω_(i). The i^(th) output of the k^(th) multichannel receiver (receivingthe signal from the k^(th) of m_(r) antennas) has the following form:##EQU1## where α_(k) (ω_(i), θ_(j) ^(i)) is the gain and phase responseof the k^(th) antenna and multichannel receiver, for the i^(th) channelto a signal arriving from θ_(j) ^(i), and

n_(k) ^(i) (t) is an unwanted noise term which incorporatesimperfections in the antennae and receiving equipment, interferingsources, and noise.

Collecting the i^(th) outputs of the m_(r) multichannel receivers¹(receiving the signals from the m_(r) antennas) in a vector, thefollowing equation is obtained: ##EQU2##

where

x_(r) (ω_(i), t)=[χ₁ (ω_(i), t), . . . , χ_(m).sbsb.r (ω_(i), t)]^(T),

a_(r) (ω_(i), θ_(j) ^(i))=[α₁ (ω_(i), θ_(j) ^(i)), . . . , α_(m).sbsb.r(ω_(i), θ_(j) ^(i))]^(T),

n^(i) (t)=[n₁ ^(i) (t), . . . , n_(m).sbsb.r^(i) (t)]^(T).

The discussion above concerns the mathematical description of thesignals received at the base station in the SDMA system. The equationsdescribing transmission from the base station in the SDMA system havemuch the same structure. The transmitting array is composed ofm_(t).sbsb.χ transmitting elements. The modulated signal input to thek^(th) transmitter in a channel, ω_(i), is denoted χ_(t).sbsb.χ^(k)(ω_(i), t). This signal enters the k^(th) transmitter, is spatiallymultiplexed with the other channels, up-converted to the carrierfrequency, and transmitted by the k^(th) antenna. Due to transmitter andantenna characteristics, the baseband sisal in channel ω_(i), astransmitted by the antenna is a function of the direction in which it isbroadcast in the medium

    y.sub.t.sbsb.χ.sup.k (ω.sub.i, θ, t)=α.sub.t.sbsb.χ.sup.k *(ω.sub.i, θ)χ.sub.t.sbsb.χ.sup.k (ω.sub.i, t),  (3)

where α_(t).sbsb.χ^(k) (ω_(i), θ) denotes the gain and phasecharacteristics of the k^(th) antenna and transmitter for the i^(th)channel as a function of the direction θ. Collecting the i^(th) inputsto the m_(t).sbsb.χ transmitters (feeding the m_(t).sbsb.χ antenna) in avector, and adding together the contributions from all the basebandsignals in channel ω_(i), the following equation is obtained: ##EQU3##where x_(t).sbsb.χ (ω_(i),t)=[χ₁ (ω_(i), t), . . . , χ_(mt).sbsb.χ(ω_(i), t)]^(T),

a_(t).sbsb.χ^(T) (ω_(i), θ)=[α₁ (ω_(i), θ), . . . , α_(mt).sbsb.χ(ω_(i), θ)].

Different channels are from this stage treated separately. The sameprocessing (cf. FIG. 4) takes place for each channel, ω_(i). Thus, theindex i is suppressed in the following discussion, and equation (2) canbe written as follows: ##EQU4## where A_(r) =[a_(r) (θ₁) . . . a_(r)(θ_(d))] and s_(r) (t)=[s_(r) (θ₁,t), . . . , s_(r) (θ_(d),t)]^(T), andequation (4) can be written as follows:

    y.sub.t.sbsb.χ (θ, t)=a*.sub.t.sbsb.χ (θ)x.sub.t.sbsb.χ (t).                          (6)

FIG. 4 shows a block diagram of the SDMA processor for one channel. Theoutput of the receiver block (FIG. 4, 42 and FIG. 7, 102,104,106) isx_(r) (t). This signal is an input to the Spatial Division MultipleAccess signal Processor (SDMAP) (FIG. 5, 48).

A model of the vector of gain and phase characteristics for receive,a_(r) (θ), and transmit, a*_(t).sbsb.χ (θ) is assumed known for θ in therange of interest. The SDMAP exploits this information together withknown properties of s_(r) (θ_(j), t), e.g., training sequences andconstant modulus properties, to:

1. appropriately compress the incoming data (FIG. 5, 160),

2. estimate the number of signals present in the channel (FIG. 5, 164),

3. estimate the directions-of-arrival (DOAs) of the incoming wavefrontsand other signal parameters (FIG. 5, 170),

4. track the locations of the users in the channel (FIG. 5, 174),

5. estimate the spatial correlation structure of the received signals,ε{s(t)s(t)*}, (FIG. 5, 180).

6. compute an appropriate spatial demultiplexing scheme (FIG. 5, 180)based on the estimates above and information from the SDMA controller(FIG. 5, 72), and appropriately set the spatial demultiplexer (FIG. 5,46, and FIG. 7, 46) so that individual incoming signals can be separatedas shown in FIG. 6, 50, and

7. compute an appropriate spatial multiplexing scheme (FIG. 5, 180)based on the estimates above and information from the SDMA controller(FIG. 5, 72), and appropriately set the spatial multiplexer (FIG. 5, 66,and FIG. 9, 66) so that the multiple transmitted signals do notinterfere with each other at the intented receive sites as shown in FIG.8.

The spatial demultiplexer (FIG. 4, 46) takes as input the output of thereceivers, x_(r) (t) and the demultiplexing scheme computed in theSDMAP. The multichannel output of the demultiplexer contains an estimateof the baseband signals of the d spatial channels s_(r) (θ_(j), t), j=1,. . . , d, obtained by combining the receiver outputs in an appropriatemanner to pass the desired signal while coherently cancelling undesiredusers in the same channel. Furthermore, the relative amount ofbackground noise is decreased in the spatial multiplexer, thus enhancingoutput signal quality compared to prior art. The individual basebandsignals are passed on to standard signal demodulators, (FIG. 4, 52)which demodulate and equalize the messages as in done in the currentstate-of-the-art.

The spatial demultiplexer is implemented either in analog or digitalhardware. In an analog embodiment, the analog-to-digital (A/D)conversion takes place in the SDMAP, and in the digital embodiment, theA/D conversion takes place in the receivers. The spatial demultiplexingis performed either in analog or digitally, and the appropriate A/D orD/A conversion of the baseband signals takes place to interface with thesignal demodulators.

The spatial multiplexer (FIG. 4, 66) takes as input the baseband messagesignals from the signal modulators (FIG. 4, 62) and the multiplexingscheme computed in the SDMAP (48). The multichannel output is spatiallymultiplexed so that when temporally multiplexed, up-converted, andtransmitted through the transmitting array, the message intended for theuser in direction θ_(j) is:

1. coherently added in the direction of θ_(j),

2. coherently cancelled in the directions of the other users of the samechannel, and

3. minimized in all other directions.

The spatial multiplexer does the above simultaneously for all spatialchannels, θ_(j), j=1, . . . , d. Thus, y_(t).sbsb.χ (θ,t) in equation(6) is equal to s_(t).sbsb.χ (θ_(j),t) for θ=θ_(j), j=1, . . . , d byappropriate choice of multiplexing scheme when forming x_(t).sbsb.χ (t).

The spatial multiplexer can be implemented using either analog ordigital techniques. Appropriate A/D and D/A conversion of theinput/output is performed to interface with the signal modulator andtransmitters.

Details of a Particular Instantiation of the SDMA Invention

To exemplify the SDMA procedure, detailed examples of the differentsteps taken are given below.

Data Compression

In one embodiment, data compression is accomplished by forming acovariance matrix from the received data ##EQU5## where N is the numberof data vectors (or snapshots), x(t_(k)), used. Spatial smoothing and/orforward-backward averaging, both well-known in prior art, are performedas appropriate. These operations can be mathematically described by atransformation of R given by:

    R.sub.T =JRJ*.                                             (8)

The signal and noise subspaces, E_(s) and E_(n), are calculated usingwell-known mathematical techniques such as eigendecompositions (EVDs)and singular-value decompositions (SVDs) ##EQU6## where

    Σ.sub.n =JJ*                                         (10)

    E.sub.s =[e.sub.1, . . . , e.sub.d ]                       (11)

    Λ.sub.s =diag[λ.sub.1, . . . , λ.sub.d ](12)

    E.sub.n =[e.sub.d+1, . . . , e.sub.m.sbsb.r ]              (13)

    Λ.sub.n =diag[λ.sub.d+1, . . . , λ.sub.m.sbsb.r ].(14)

These equations describe the processing of a block of data, i.e., batchmode. Alternatively, the data can be recursively processed with thequantities above updated as new data become available. Such techniquesare well-known in prior art.

Signal Detector

In one embodiment, signal detection is accomplished using statisticalcriteria such as Minimum Description Length (MDL), An InformationCriterion (AIC), or Weighted Subspace Fitting (WSF) detection, allwell-known in prior art. Information from the SDMA controller pertainingto the number of sources locally assigned to that particular channel isalso used in the detector to set a lower bound on the estimated numberof signals present.

Signal Parameter Estimator

In one embodiment, a Maximum Likelihood estimator is employed to obtainthe signal parameter estimates, θ, the emitter signal covarianceestimate, S, and the noise variance estimate, σ². These are obtained byminimizing the following cost function

    V(θ, S, σ.sup.2)=log|R|+NTr{R.sup.-1 R.sub.T }(15)

    R=JA(θ)SA*(θ)J*+σ.sup.2 JJ*.             (16)

Techniques for performing the minimization are well-known in prior art.In other embodiments, algorithms making use of signal and noisesubspaces may be used to estimate the signal parameters.

Source Tracker

In one embodiment, an extended Kalman filter (EKF) takes DOA estimatesfrom the DOA estimator as inputs, and outputs estimates of the kinematicstate of the transmitter, i.e., its position and velocity as a functionof time. Such filters are well-known in prior art (Chui, op. cit.). Inanother embodiment, DOA estimates from a plurality of base stationsreceiving signals from a plurality of users are processed in an EKE inthe SDMA controller to obtain location estimates of the users and thelocation estimates are communicated back to the base stations by theSDMA controller. In yet another embodiment, time-of-arrival (TOA)information obtained at the base stations from known signal propertiesusing techniques well-known in the current state-of-the-art and asdescribed previously, is used in addition to DOA estimates to estimatethe user locations. In general, the embodiment which exploits all theinformation available related to the location of the transmitter is tobe preferred. The capability to locate users using such measurementsmade at base stations is unique to this invention.

Demultiplexer Controller

In one embodiment, an appropriate collection of weights W_(r) =[w_(r)(θ₁) . . . w_(r) (θ_(d))] is calculated, one set, w_(r) (θ_(k)), foreach signal, s_(r) (θ_(k), t), to be demultiplexed. The calculation ofthe appropriate weights in this embodiment involves noise covariance andsignal correlation estimation from which robust structured-stochasticsignal copy weights are calculated as follows:

    W.sub.r =(ASA*+σ.sup.2 I+D(Σ.sub.CRB ⊙S)D*).sup.-1 (AS+D'(Σ.sub.CRB ⊙S).                  (17)

where

    A=A(θ)                                               (18)

    S=(A*(θ)A(θ)).sup.-1 A*(θ)(R-σ.sup.2 I)A(θ)(A*(θ)A(θ)).sup.-1                (19) ##EQU7##

    Σ.sub.CRB =Cramer-Rao bound on θ               (23)

and ⊙ denotes element-wise multiplication.

Multiplexer Controller

In one embodiment, the same mathematical formulae used by thedemultiplexer controller to calculate the demultiplexing weights, areused to calculate the appropriate collection of multiplexing weights,W_(t).sbsb.χ =[w_(t).sbsb.χ (θ₁) . . . w_(t).sbsb.χ (θ_(d))]. In eachset, w_(t).sbsb.χ (θ_(k)), a weight, w_(t).sbsb.χ^(k) (θ_(k)), k=1, . .. , m_(t).sbsb.χ, is computed for each of the m_(t).sbsb.χ transmittingantennas. One set of weights is computed for each signal to betransmitted.

Demultiplexer

In one embodiment, the spatial demultiplexing of signal s_(r) (θ_(k), t)is achieved by multiplying the output of the receivers, x_(r) (t), bythe appropriate weight, w_(r) (θ_(k)), then obtaining their sum

    s.sub.r (t)=W*.sub.r x.sub.r (t).                          (24)

This process is hereafter referred to as signal copy.

Multiplexer

In one embodiment, the spatial multiplexing of the transmission signals,s_(t).sbsb.χ (θ_(k), t), is achieved by multiplying the individualsignals, s_(t).sbsb.χ (θ_(k), t), by the appropriate set of multiplexingweights, w_(t).sbsb.χ (θ_(k)). The resulting spatially modulated signalhas the form

    x.sub.t.sbsb.χ (t)=W.sub.t.sbsb.χ s.sub.t.sbsb.χ (t)(25)

where s_(t).sbsb.χ (t)=[s_(t).sbsb.χ (θ₁,t), . . . , s_(t).sbsb.χ(θ_(d), t)]^(T).

SDMA Controller

A function of SDMA controller (72) is to prevent wireless units frombecoming coincident in (frequency or code) channel, time, and spatial(location) space. As required, the controller instructs the wirelessunits to change to different (frequency or code) channels via standardmessaging schemes present in current state-of-the-art wireless systems.In one embodiment, this is performed by calculating a weighted measureof the proximity of all users in the cell. Pairwise user spatiallocation separations (i.e., DOA differences) are weighted inverselyproportional to the maximum receiving antenna array beamwidth at the twoDOAs, and the frequency difference measure is binary-valued, taking thevalue 1 if the frequencies are different and 0 if they are the same.

Denoting the channel assigned to user i by ω_(i), its DOA by θ_(i) (t),and the array beamwidth at DOA i by θ_(i) ^(BW) (t), a distance measureD_(ij) (t) can be written as follows: ##EQU8## where δ(ω_(i),ω_(j)) is 1if ω_(i) =ω_(j) and 0 if ω_(i) ≠ω_(j). When D_(ij) (t)<γ for any pair ofusers {i,j}, where γ in one embodiment is a fixed constant near unity, afrequency reallocation is performed by finding

    max.sub.i,j,k {max(D.sub.ik (t.sub.s), D.sub.kj (t.sub.s))},(27)

where t_(s) is the time at which D_(ij) (t)<γ. That is, the value of kthat maximizes the new distance measure is selected and the appropriateuser, either i or j, is switched to channel ω_(k) for transmitting tothe base station. This same algorithm is employed to select channels inwhich base stations transmit to users by simply replacing receiveantenna array parameters with transmit antenna array parameters inequation 27. In another embodiment, signal strength and direction oftravel are used to develop more robust switching strategies. In evenmore sophisticated embodiments, a similar optimization is performedusing information from a plurality of base stations at a base stationsupervisor to allocate transmit and receive channels among the multiplebase stations and multiple users comprising the wireless system.

SUMMARY AND SCOPE OF THE INVENTION

Thus, it is clear to see that the present invention is a method andapparatus for increasing capacity and improving quality of wirelesscommunication networks. The locations of multiple sources simultaneouslytransmitting information in a common channel can be estimated and theindividual signal waveforms reconstructed. Information is simultaneouslytransmitted to the sources on a common channel without creatingcochannel interference which would otherwise compromise two-way(full-duplex) communication links. Furthermore, the invention providesfor tracking of mobile sources, mitigating the hand-off and signalmangement problems and is compatible with current and future modulationschemes in wireless communication systems.

While the above description contains certain specificities, these shouldnot be construed as limitations on the scope of the invention, butrather as an exemplification of one preferred embodiment and applicationthereof. Many other variations are possible. For example, the systemcan:

1. be used to monitor the quality of service provided by proposed cellsite locations,

2. be used to increase security by transmitting signals only inpreferred directions, thereby also limiting the amount of undesiredradiation,

3. be implemented on mobile units, thereby endowing the mobile unitswith many of the aforementioned advantages, and additionally providingthe capability for point-to-point service where mobile units transmitand receive directionally to and from each other.

Accordingly, the scope of the invention should not be determined by theembodiments illustrated, but by the appended claims and their legalequivalents.

What is claimed is:
 1. A wireless systems for receiving signals transmitted by at least one transmitter at locations remote from at least one base station, said system comprising in combination:receiving means, including plural receiving antennas for making measurements of received signals received by said base stations, the received signals resulting from signals transmitted by at least one transmitter, said measurements being composed of different combinations of said transmitted signals, processing means for estimating the number of said received signals and estimating parameters thereof, and spatial demultiplexing means for obtaining said transmitted signals from said measurements by separating said received signals using said estimated number of received signals and said estimated parameters of said received signals, and associating said received signals with each of said at least one transmitter, whereby the number of received signals, parameters thereof, and said signals transmitted by each transmitter become known.
 2. A wireless communication system for receiving signals transmitted by spatially distributed users, said system comprising:receiving means, including a plurality of spatially distributed receiving antennas for making measurements of received signals, said received signals resulting from signals transmitted by said users, said measurements made by each said antenna comprising different combinations of said transmitted signals; processing means including statistical analysis means for estimating the number of said received signals by statistical analysis, and parameter estimating means for estimating parameters of said received signals; characterized in that said received signals result from signals transmitted by said users in a common communication channel; in that said parameter estimating means is arranged to estimate parameters of said received signals using predetermined or learnt information on said receiving antennas; and by spatial demultiplexing means for obtaining said transmitted signals from said measurements using said estimated number of received signals and said estimated parameters of said received signals by associating respective said received signals with respective said users.
 3. The wireless communication system of claim 2 wherein said receiving means comprises plural multichannel receivers for obtaining said measurements from said receiving antennas for plural channels, each multichannel receiver having one channel for each of said plural channels.
 4. The wireless communication system of claim 2 wherein:said statistical analysis means comprises means for determining the apparent number of said received signals by employing statistical methods using eigenvalues of a covariance matrix calculated using said measurements; said parameter estimating means comprises means for determining the actual number of said received signals, from said measurements using optimization means for obtaining said parameters from a suitable criterion based on said measurements and a model of said measurements in terms of said parameters; and said processing means includes estimating means for tracking locations of said users from said parameters and said measurements.
 5. The wireless communication system of claim 4 wherein said spatial demultiplexing means includes:means for combining said received signals associated with each of said users.
 6. The wireless communication system of claim 3 wherein the system comprises a distributed plurality of said receiving means, said processing means and said spatial demultiplexing means; andmeans for assigning each of said users to one of said channels for selecting at least one of said spatial demultiplexing means for spatially demultiplexing said measurements to obtain said transmitted signals in each of said channels.
 7. A wireless communication system for transmitting signals to a plurality of spatially distributed users, said system comprising:spatial multiplexing means for spatially multiplexing signals to be transmitted to said users in each channel of a plurality of channels to form a plurality of different signal combinations for each said channel; and transmitting means, including plural transmitting antennas and plural multichannel transmitters for transmitting said spatially multiplexed signals to said users, said transmitting means having one said multichannel transmitter for each said transmitting antenna with one channel of each said multichannel transmitter for each said channel, each said transmitting antenna being arranged to receive a different said signal combination such that said signals in each said channel are transmitted to said users in a spatially directive manner.
 8. The wireless communication system of claim 7 wherein said system comprises a distributed plurality of said spatial multiplexing means and said transmitting means; andmeans for selecting with respect to each of said signals to be transmitted to said users, a particular said transmitting means from which to transmit said signals to said users; and means to select constituent signals for said signal combinations, one for each of said transmitting antennas, using said locations of said users and parameters of said transmitting antennas, and for controlling said multiplexing means to generate signals to be transmitted by each of said transmitting antennas by combining said constituent signals in each of said channels.
 9. The wireless communication system of claim 8 further comprising:means for assigning each of said users to one of said channels.
 10. A wireless communication system for transmitting signals to and receiving signals from spatially distributed users, said system comprising:receiving means, including a plurality of spatially distributed receiving antennas and a plurality of receivers for at least one receive channel and for making measurements of received signals in each of said receive channels, said received signals resulting from signals transmitted by said users, said measurements made by each said antenna comprising different combinations of said transmitted signals in each of said receive channels; processing means including statistical analysis means for estimating the number of said received signals by statistical analysis, and parameter estimating means for estimating parameters of said received signals; characterized in that said received signals result from signals transmitted by said users in a common communication channel; in that said parameter estimating means is arranged to estimate parameters of said received signal using predetermined information on said receiving antennas; and by: spatial demultiplexing means for obtaining said transmitted signals in each of said receive channels from said measurements said number of received signals and said parameters by associating respective said received signals with respective said users; spatial multiplexing means for spatially multiplexing signals to be transmitted to said users in at least one transmit channel using said parameters, to form a plurality of different signal combination for each said transmit channel; and transmitting means including plural transmitting antennas and plural multichannel transmitters for transmitting said spatially multiplexed signals to said users, said transmitting means having one said multichannel transmitter for each said transmitting antenna, with one channel of said multichannel transmitter for each said transmit channel, each said transmit channel being arranged to receive a different said signal combination such that said signals in each said channel are transmitted to said users in a spatially directive manner; said system being arranged such that reception of multiple signals in multiple channels from said users and transmissions of multiple signals in multiple channels to said users take place simultaneously, thus establishing plural full-duplex communication links, one for each of said users.
 11. The wireless communication system of claim 10 wherein:said statistical analysis means comprises means for determining the apparent number of said received signals by employing statistical methods using eigenvalues of a covariance matrix calculated using said measurements, said parameter estimating means comprises means for determining the actual number of said received signals, from said measurements using optimization means for obtaining said parameters from a suitable criterion based on said measurements and a model of said measurements in terms of said parameters; and said processing means includes estimating means for tracking locations of said users from said parameters and said measurements.
 12. The wireless communication system of claim 10 wherein said spatial demultiplexing means includes means for combining said received signals associated with each of said users.
 13. The wireless communication system of claim 10 wherein said system comprises a distributed plurality of said spatial multiplexing means, and said transmitting means; andmeans for selecting with respect to each of said signals to be transmitted to said users, a particular said transmitting means from which to transmit said signals to said users, and means for selecting constituent signals in said transmit channel for said signal combinations one for each of said transmitting antennas, using said locations of said users and parameters of said transmitting antennas, and for controlling a said multiplexing means to generate signals to be transmitted by each of said transmitting antennas by combining said constituent signals in each of said channels.
 14. The wireless communication system of claim 10 wherein said system comprises a distributed plurality of said receiving means, said processing means and said spatial demultiplexing means; andmeans for assigning each of said users to one of said receive channels, and for selecting at least one of said spatial demultiplexing means for spatially demultiplexing said measurements to obtain said transmitted signals in each of said receive channels.
 15. The wireless communication system of claim 10 further comprising:means for assigning each of said users to one of said transmit channels, and for selecting at least one of said multiplexing means for spatially multiplexing said signals to be transmitted to said users in said transmit channels. 